Solidly fills a rectangular area (cuboid) with blocks of a given type. After entering the command you need to mark 2 opposing corners of the area. You can do that either by clicking the blocks (building or deleting works), or using the /Mark command. The results of /Cuboid can be undone using /Undo.

/Cuboid

/Cuboid BlockType

This information only applies when command is used with the Normal brush.

Fills the area with a given block type.

Drawing lines with /Cuboid

Drawing rectangles and squares (2D) with /Cuboid

Drawing cuboids and cubes (3D) with /Cuboid

Customizing /Cuboid behavior

The volume of blocks that can be drawn at once can be limited per-rank. See "Draw limit" field on ConfigGUI's Ranks tab. If you are editing config.xml manually, set the "drawLimit" attribute on the <Rank> tag.

Being able to undo a command requires storing a backup of an area of the map. This may take up significant amounts of memory. By default, fCraft limits /Undo to the first 2,000,000 blocks that are drawn. You can change this (or remove the limit entirely) via MaxUndo config key.